Additive manufacturing enables a component to be fabricated by building it in layers. When applied to the manufacture of metallic or ceramic objects, each layer is melted, sintered, or otherwise integrated onto a previous layer such that each layer may be modeled as a slice or sectional plane of the final object. For example, selective laser melting (SLM) and selective laser sintering (SLS) have been used to build components layer by layer from powder beds. In these processes a powder bed of a component final material, or a precursor material, is deposited onto a working surface, and then laser energy is directed onto the powder bed following a cross-sectional area shape of the component to create a layer or slice of the component. The deposited layer or slice then becomes a new working surface for the next layer.
While SLM and SLS are generally limited to flat working surfaces, laser microcladding is a 3D-capable process that deposits a small, thin layer of material onto a surface by using a laser beam to melt a flow of powder directed towards the surface of an object. In laser microcladding the powder is propelled towards the surface by a jet of gas, and when the powder is a metallic material the gas is usually a protective inert gas, such as argon, which is capable of shielding the resulting molten metal from atmospheric oxygen. Laser microcladding is limited, however, by its low deposition rate which ranges from about 1 to 6 cm3/hr. Furthermore, because the protective gas tends to dissipate before the clad material is fully cooled, superficial oxidation and nitridation may occur on the surface of the deposit. Such impurities can be especially problematic when multiple layers of clad material are necessary to achieve a desired cladding thickness.
A similar problem also tends to occur when superalloy components are fabricated using SLM or SLS. Even when melted superalloy materials are shielded from the atmosphere by applying inert gases such as argon during laser heating, these processes tend to trap oxides (e.g., aluminum and chromium oxides) within the layer of deposited material—resulting in porosity, inclusions and other mechanical defects (e.g., cracking) associated with the trapped oxides. To mitigate this problem, post-deposition processes such as hot isostatic pressing (HIP) have been used to collapse these voids, inclusions and cracks in order to improve the thermal and mechanical properties of the deposited coating.
It has been proposed to employ SLM and SLS of static beds of powdered metal alloys in order to manufacture superalloy components by additive manufacturing. However, components produced using these techniques have been limited due to low productivity and quality. Use of static beds of powdered materials greatly limits productivity because the incrementally-deposited layers tend to be very thin. Moreover, the interface between incrementally processed layers or planes is often subject to defects and questionable physical properties. Use of mixed bed approaches also does not allow for selective placement of different materials to form integrated systems containing multiple materials. Such integrated systems may include, for example, an inner superalloy substrate coated with a diffusion bonded MCrAlY coating which is further bonded to an outer ceramic thermal barrier coating (TBC).
Selective placement of different materials would be necessary in order to employ laser additive manufacturing (LAM) techniques to efficiently produce multi-material components containing integrated systems such as the gas turbine airfoil 20 illustrated in FIG. 1. FIG. 1 is a cross-sectional view of an exemplary gas turbine airfoil 20 containing a leading edge 22, a trailing edge 24, a pressure side 26, a suction side 28, a metal substrate 30, cooling channels 32, partition walls 34, turbulators 36, film cooling exit holes 38, cooling pins 40, and trailing edge exit holes 42. In this example, whereas the metal substrate 30, partition walls 34, turbulators 36 and cooling pins 40 are fabricated of a superalloy material, the exterior surfaces of the airfoil substrate 30 are coated with a porous ceramic thermal barrier coating 44. A metallic bond coat 45 such as an MCrAlY may also be applied between the superalloy substrate 30 and the thermal barrier coating 44 to enhance bonding between the superalloy and ceramic layers and to further protect the superalloy material from external oxidants.
Thus, use of LAM techniques to produce a multi-material component such as the airfoil 20 of FIG. 1 would require not only the selective placement of different materials, but it would also require an ability to selectively apply different processing conditions (i.e., placement and intensity of laser heating) to these different materials. This is because selective melting of a superalloy powder to form the metal substrate 30 would generally require different heating conditions than selective sintering of a ceramic powder to form the thermal barrier coating 44. Another serious complication arises from the need to protect the superalloy powder and resulting metal substrate 30 from reacting with atmospheric oxidants such as oxygen and nitrogen. Especially for a large airfoil 20, the use of LAM techniques could also require an ability to perform SLM and SLS under atmospheric conditions without jeopardizing the chemical and/or physical properties of the resulting component.